September 2023

Networking explained with a horse and carriage

Imagine you're a king or queen ruling over a vast realm.

Key to your power is knowledge. You've maintained a network of horse riders and stables for your messengers to use as they bring news from far parts of the kingdom to you.

One day, you have an incredible proclamation, a wedding invitation for the grandest ceremony and party.


You pay several local villagers to ride their horse to a city in the realm, each carrying a "save the date" letter. Since the date is far off you tell the messengers that their job is done after delivering the message.

This would be analogous to sending UDP packets. Messages are sent without confirmation of delivery. After giving these instructions to the riders you realize you don't know when or if the letters arrived at their destination.

After sending the letters you get some unfortunate news. One of the most important guests cannot attend that date, and so the wedding date will have to be changed. You repeat the process, sending out messengers with the new date.

Some of the letters with the new date arrive sooner than the letters with the old dates. You end up getting confused envoys asking which date is the real date. This is known in computer networking as

Determined not to create any more chaos you hire the wisest thinkers to implement a system that can deliver messages in order reliabily. Here is a system they came up with. They call it TCP.


You use a trusted courier that will deliver the message, then return with confirmation that the message has been delivered. Easy enough. Your thinkers also tell you to write down a number on each message corresponding to the number of letters sent to that destination. That way the recipient knows if a message comes out of order... And they will also know if they've missed a message when they see a gap in numbers from letters they received.

TCP is like this trusted courier. TCP is a protocol that provides reliable, ordered delivery of data. If there's a gap in the messages, the recipient can request another messenger from the sender in order to get a copy of that missing message.

Some computer terminology: The time it takes for the messenger to travel from the capital to its destination is called Latency. Instead of messengers you would call them "packets". Ping would be how long it takes for a full round-trip. In this analogy some of the messages may be time-sensitive (especially as the wedding date gets closer). If a message doesn't arrive to its destination for any reason (bandits? treacherous cliffsides?), it might not be noticed until the next message is received, and then it would still require a messenger to be sent back to the capitol in order to request a copy of the missing of letters. This increased number of round-trips and additional latency is one of the big reasons TCP is not a great system for fast communication (like in fast paced games).

In real life before trains, latency would measure in days, weeks, or even months for very far off destinations. But in computer networks, latency is anywhere between a few milliseconds to... well, actually there is no upper limit.

Light could theoretically circumnavigate the Earth within 133 milliseconds, which means our best fiber optic cables could feasibly see latency as low as 66ms for somebody on the other side of the world. This number is optimistic because there's always slow down from hops along different routers to and along the internet backbone, and from wifi or satellite. There can also be lots of internet "traffic" which bottlenecks some routes around the internet causing lag spikes.

When you navigate to a website or play an online game, you can imagine signals traveling back and forth between your computer and the server that hosts that website or game. Every time you see the page load or the contents update, some data must have been exchanged. Imagine if a message is lost because a line went dead or one of the many routers on the route had a bug or reset. In UDP those messages would be lost forever. With TCP, there is a way for the sender and receiver to request the missing messages but with a big latency penalty equal to the round trip time. The basic principles of message passing still are followed in the same way they were when the word was spread by horse and carriage. Many complex networking related topics like the [](Cap Theorem) can be explained by the horse and carriage analogy – because at its core it boils down to simple message passing.